To meet the demands of the high performance of electronic systems such as personal computers or electronic communication devices, nonvolatile semiconductor memory devices such as DRAMs mounted as memory devices have been designed to operate at higher speed and have become highly integrated. Since semiconductor memory devices mounted on battery-driven systems such as mobile phones or notebook computers desperately require a low power consumption characteristic, many efforts and developments have been made to reduce an operating current and a standby current.
A data retention characteristic of a DRAM cell consisting of one transistor and one storage capacitor is very sensitive to temperature. Therefore, it may be necessary to adjust the operating conditions of circuit blocks inside semiconductor integrated circuits, depending on the variation in an ambient temperature. For example, DRAMs used in mobile products adjust a refresh period, depending on the variation in an ambient temperature. Temperature sensors, such as a Digital Temperature Sensor Regulator (DTSR), an Analog Temp Sensor Regulator (ATSR), and a Digital Temperature Compensated Self Refresh (DTCSR), are used to adjust the operating conditions depending on the variation in the ambient temperature.
A DTCSR is a temperature sensor which is used to sense an ambient temperature and determine a refresh period of memory cells. A typical DTCSR senses an ambient temperature to generate a variable voltage, and compares the level of the variable voltage with the level of a reference voltage to generate a code signal. In the DTCSR, the variable voltage is generated from a circuit which is implemented with an NMOS transistor, a PMOS transistor, a resistor, and so on. Therefore, the variable voltage changes nonlinearly with the temperature variation, and it is difficult to predict the voltage variation which is caused by the process variation. Furthermore, it is difficult to measure the temperature exactly because the voltage variation has a gentle slope with respect to the temperature voltage.